Various configurations of exhaust emission control catalysts disposed in an exhaust system of an automobile are known. However, the usual exhaust emission control catalyst exhibits a purifying ability by oxidation in a high temperature range of an exhaust gas, e.g., about 180.degree. C. or more, and exhibits an extremely low purifying ability with respect to an exhaust gas having a low temperature immediately after an engine is started and containing HC at a high concentration.
Therefore, to catch HC in the low-temperature exhaust gas, it is contemplated that an HC adsorbent is disposed in the exhaust system downstream of the catalyst. In this case, zeolite is used as the HC adsorbent.
The temperature of the exhaust gas flowing in the exhaust system is generally on the order of 1,000.degree. C., and for this reason, a heat resistance accommodating such temperature is required for the HC adsorbent.
There has been proposed a zeolite having a high silica/alumina, ratio and containing a reduced amount of aluminum which becomes a starting point of thermal deterioration, in order to enhance the heat resistance. However, even the zeolite suffers from a problem that the heat resistance thereof is not improved sufficiently, and the HC adsorbing and desorbing ability is also low.